Long Term Evolution (LTE) Advanced is a mobile telecommunication standard proposed by the 3rd Generation Partnership Project (3GPP) and first standardised in 3GPP Release 10. In order to provide the peak bandwidth requirements of a 4th Generation system as defined by the International Telecommunication Union Radiocommunication (ITU-R) Sector, while maintaining compatibility with legacy mobile communication equipment, LTE Advanced proposes the aggregation of multiple carrier signals in order to provide a higher aggregate bandwidth than would be available if transmitting via a single carrier signal. This technique of Carrier Aggregation (CA) requires each utilised carrier signal to be demodulated at the receiver, whereafter the message data from each of the signals can be combined in order to reconstruct the original data. Carrier Aggregation can be used also in other radio communication protocols such as High Speed Packet Access (HSPA).
Carrier signals are typically composed of a carrier frequency that is modulated to occupy a respective radio frequency carrier signal band. Contiguous Carrier Aggregation involves aggregation of carrier signals that occupy contiguous radio frequency carrier signal bands. Contiguous radio frequency carrier signal bands may be separated by guard bands, which are small unused sections of the frequency spectrum designed to improve the ease with which individual signals can be selected by filters at the receiver by reducing the likelihood of interference between signals transmitted in adjacent bands. Non-contiguous Carrier Aggregation comprises aggregation of carrier signals that occupy non-contiguous radio frequency carrier signal bands, and may comprise aggregation of clusters of one or more contiguous carrier signals. The non-contiguous radio frequency carrier signal bands are typically separated by a frequency region which is not available to the operator of the network comprising the carrier signals, and may be allocated to another operator. This situation is potentially problematic for the reception of the carrier signals, since there may be signals in the frequency region that separates the non-contiguous carriers which are at a higher power level than the wanted carrier signals.
A Direct Conversion Receiver (DCR) is typically employed to receive cellular radio signals, and typically provides an economical and power efficient implementation of a receiver. A DCR uses a local oscillator placed within the radio frequency bandwidth occupied by the signals to be received to directly convert the signals to baseband. Signals on the high side of the local oscillator are mixed to the same baseband frequency band as signals on the low side of the local oscillator, and in order to separate out the high and low side signals, it is necessary to mix the signal with two components of the local oscillator in quadrature (i.e. 90 degrees out of phase with one another) to produce inphase (I) and quadrature (Q) signal components at baseband. The I and Q components are digitised separately, and may be processed digitally to reconstruct the separate high side and low side signals. The reconstructed high and low side signals may be filtered in the digital domain to separate carrier signals received within the receiver bandwidth of the DCR.
The presence of a higher power signal in the region separating non-contiguous carrier clusters poses particular problems if a DCR is to be used to receive a band of frequencies comprising non-contiguous Carrier Aggregation signals. In particular, since the higher power signal is within the receiver bandwidth, the dynamic range of the receiver need to encompass the powers of the wanted carrier signals, which are typically received at a similar power to each other, and the higher power signal. This may place severe demands on the dynamic range of the analogue to digital converter (A/D) in particular. Furthermore, due to inevitable imbalances between the amplitudes and phases of the I and Q channels, the process of reconstructing the separate high side and low side signals suffers from a limited degree of cancellation of the image component; that is to say, some of the high side signals break through onto the reconstructed low side signals, and vice versa. The degree of rejection of the image signal may be termed the Image Reject Ratio (IRR). If the higher power signal is a high side signal, it may cause interference to received low side signals due to the finite IIR, and similarly if the higher power signal is a low side signal, it may cause interference to received high side signals.
One conventional method of receiving Non-contiguous Carrier Aggregation signals is to provide two DCR receiver stages, each having a local oscillator tuned to receive a cluster of contiguous carriers, and so rejecting signals in the frequency region between the clusters before digitisation. However, this approach is potentially expensive and power consuming, and may suffer from interference between the closely spaced local oscillators.
It is an object of the invention to address at least some of the limitations of the prior art systems.